The present invention utilizes GPS or other Global Navigation Satellite System (GNSS) orbit predictions to seed orbit propagators embedded in mobile devices with a parameter set that provides the capability to propagate GPS orbits with the fidelity required for accurate navigation in mobile devices. The accuracy of the predictions is limited by the fidelity of the propagator implemented within the mobile device and the inability to precisely model the forces acting on the GPS satellites. The use of GPS to describe the current invention is for convenience only. For the purposes of this invention the invention described applies equally to other GNSS such as GLONASS and Galileo or other future or planned satellite based positioning systems. The context of a mobile client to describe the current invention is for convenience only. For the purposes of this invention, the invention described applies equally to any remote clients, whether mobile or fixed.
Satellite positioning and prediction is used extensively in many applications Notably, users of the GPS require the positions of the GPS satellites at the time the ranging signal was transmitted in order to compute a position solution. Currently this orbit information is provided by the satellites on the radio frequency (RF) data link in the form of a satellite position model. The model utilizes a Keplerian-like set of orbital elements, termed the ephemeris, which is valid for a limited time period, typically 4 to 6 hours. The GPS satellites broadcast the ephemeris data and the receiver demodulates the ephemeris data from the RF data link that is continuously monitoring this data stream for an updated ephemeris. The ephemeris data is a model that allows the user to evaluate a set of equations, the mathematical model of the fit to an orbit arc, and obtain the satellite position at any time during the model fit period. Although the model allows the evaluation of the satellite position beyond the 4 to 6 hours of validity, the accuracy degrades asymptotically.
In traditional GPS applications, this method was sufficient because the GPS receivers had unobstructed clear view of the satellites and the evaluation of the ephemeris model put few demands on the computing power of the receivers. However, in modern applications, such as embedding a GPS receiver within a mobile device such as a mobile phone, the GPS receiver within the mobile device does not have unobstructed clear views of the satellites and must often work in weak signal environments. This severely limits the application of this type of distribution of satellite position models because the ephemeris model cannot be reliably received from the satellites.
Current GPS receivers in mobile devices continue to demodulate the broadcast ephemeris as much as possible similar to traditional GPS receivers to provide autonomous operation. However, because of the harsh operating environments and the requirement for rapid time to fix, these receivers are often provided with assistance or aiding data that enables rapid time-to-first-fix (TTFF) and increased sensitivity. This technique of providing assistance or aid to a GPS is commonly called Assisted or Aided GPS (AGPS). A major factor in the decreased time-to-first-fix (TTFF) and increased sensitivity is the removal of the requirement to demodulate the broadcast ephemeris data prior to direct satellite signal acquisition.
Although other types of aiding data are used, such as relative range estimates, predictions of which satellites are in view, relative satellite Doppler offsets, and time and frequency aiding, this invention is focused on the satellite orbit component of AGPS. For many AGPS systems, the broadcast ephemeris is provided from a network of GPS reference stations that provide pre-formatted reference data. This data is transformed by an AGPS server into industry standard format and then delivered to the mobile device over the mobile communications network. These GPS reference stations are located at fixed installations that have clear unobstructed views of the satellites and can therefore obtain the broadcast ephemeris and provide that broadcast data over the mobile device communications link to the embedded GPS receiver. Although this is an improvement over the GPS receiver in the mobile device demodulating the broadcast ephemeris stream, it puts undue demands on the GPS receiver within the mobile device to be continuously connected to and passing data across the mobile communications network. Additionally it requires a real-time connection between the GPS reference stations and the mobile communications network that supplies the AGPS data to the mobile. This arrangement of data flowing from the GPS reference station onto the mobile communications network, then finally reaching the GPS receiver embedded within the mobile device has several points in the data path that could be unavailable at any given instant. The present invention reduces the demands on the connectivity and data transmissions on the mobile communications network and also mitigates latencies associated with the delivery of the assistance data.
A further enhancement to aiding the GPS receiver in a mobile device is provided by making ephemeris data for future times available to the mobile device. For this process, the satellite orbits are computed using observations of satellite position, satellite velocity, range, range-rate, Doppler, or similar measurements. Once an orbit is determined from the raw data, then the future positions of the satellites is determined by propagating the orbit forward with techniques well known in the art of orbital mechanics. These orbit predictions are then used to generate so-called synthetic ephemeris in the same mathematical representation as the broadcast ephemeris. Each synthetic ephemeris will cover a 4 to 6 hour block of time for a single satellite. To extend the capability beyond the 4 to 6 hour range, multiple synthetic ephemerides are provided. For example, for a single satellite, 6 synthetic ephemerides would be provided to describe the orbit for one day. Once the synthetic ephemerides are disseminated to the GPS receiver within the mobile device, the GPS receiver's knowledge of the GPS satellite positions does not expire after 4 or 6 hours or after it loses contact with the mobile communications network. Instead the GPS receiver will look to the appropriate time block of ephemeris data.
The synthetic ephemeris can be provided for as long into the future as the orbit can be reliably predicted. Typically this prediction period is between 1 and 10 days, although longer prediction periods are possible. This synthetic ephemeris method is an improvement over the previous techniques in that there is less pressure on demodulating the broadcast ephemeris data and less real-time overhead data traffic pressure on the mobile communications network.
Another vendor, Global Locate, has developed the so called Long Term Orbits (LTO) based on centrally processed future estimates of satellite position and velocity. These position and velocity are then fit into a defined time period, typically 4-6 hours, to form a continuous time period of synthetic ephemeris. The synthetic ephemeris is constructed such that when supplied to a standard GPS receiver, it mimics standard ephemeris data A synthetic ephemeris is then locally retrieved for each 4-6 hour interval that the mobile GPS receiver is to operate without connection to the LTO data source. For example, one day of LTO data would comprise six, 4-hour, synthetic ephemeris data sets for each GPS satellite. This invention is covered under U.S. Pat. Nos. 6,560,534, 6,829,535, 6,651,000, 6,542,820, 6,703,972.
For example, one day of LTO data would comprise six, 4-hour, synthetic ephemeris data sets for each GPS satellite. However, conventional predicted GPS methods like LTO require dissemination of a large group of ephemeris data sets, typically 40 Kb to 100 Kb, over a mobile communications network, each valid for 4 to 6 hours, for each satellite. The current invention significantly reduces the volume of data required to be broadcast over the mobile communications network to the order of 2 KB or less because it does not transmit pre-generated synthetic ephemeris or future estimations of satellite positions and velocities. Rather, the current invention provides the mobile device with an initial satellite position and velocity, along with some force model coefficients, from which the mobile client software can locally propagate these satellite positions and velocities into the future and locate generate its own synthetic ephemeris data, as required to supply the GPS or AGPS device Finally, conventional predicted GPS methods like LTO are typically proprietary and designed to only work with the vendor's own AGPS chipset, leading to logistical and provisioning issues. The current invention is designed to seamlessly align with existing real-time AGPS industry standards and thus easily integrate with potentially any AGPS chipset.
Prior art techniques distribute GPS orbit information to a GPS receiver via a mathematical model of the satellite position and velocity over a relatively short period of time, typically 4-6 hours. The GPS receiver within the mobile device then evaluates that model as a function of time to determine the satellite position and velocity. The prior art techniques do not provide the GPS receiver within the mobile device the ability to propagate orbit position and velocity information with the required accuracy for position computation.
The primary purpose of the GPS ephemeris data is to recover the GPS satellite position and velocity. The process described above implements the following steps: 1) prediction of the GPS satellite position and velocity into the future, 2) reduction of that prediction to a mathematical model, 3) transferring that mathematical model of the GPS satellite position and velocity to the mobile GPS receiver, and 4) the mobile GPS receiver evaluating that mathematical model to recover the GPS satellite position and velocity. The present invention eliminates the reduction to and evaluation of the mathematical model described in steps 2) and 3).
Providing estimates of a satellite position and velocity propagated into the future is well known in the art. For the GPS satellites, this service has been provided by the International Geodetic Survey (IGS) since January of 1994. Currently the IGS provides a two day prediction of GPS satellite position and velocity, as the so-called UltraRapid product combining 2 days of data with 2 days of predictions with a position and velocity for each 15 minute interval. These data products, and other IGS data products for GLONASS, GPS clocks, etc, are available from the IGS data repository.
To compute a navigation solution using the GPS satellite orbits, the GPS receiver must interpolate between the prediction times to the exact time required for the particular navigation solution. Therefore, a high degree of granularity is required in the satellite position and velocity prediction and this is precisely what the IGS supplies. These IGS products are commonly used in post-processing where GPS range measurements are collected in the field, then returned to a central location with sufficient data storage and computing power to store and interpolate the GPS satellite position data The GPS data is not used to continue the propagation to future times. To utilize the IGS-style data products in a mobile device, a relatively large amount of orbit data would be transmitted over the mobile communications network. A major motivation for the ephemeris model, the synthetic ephemeris, and the Global Locate LTO implementation is to reduce the amount of data that must be broadcast.
The prior art techniques do not provide the GPS receiver within the mobile device the ability to propagate orbital information with the required accuracy for position computation. Further, the prior art associated with providing GPS ephemeris data to the mobile device has been constrained to the mathematical model utilized in the live data broadcast from the satellite.
Accordingly, it is an object of the invention to provide a system which reduces the amount of data required to be transmitted. It is a further object of the invention to provide a GPS receiver within the mobile device which can propagate a satellite orbit, removing the dependency of the 4 to 6 hour satellite ephemeris model. It is yet a further object of the invention to provide a GPS receiver which has the capability to propagate orbits based on the amount of unused computational power that is available, thus further reducing reliance of the GPS receiver/mobile devices on the network. It is yet another object of the invention to provide rapid updates from the mobile communications network for satellite integrity through small “maintenance data” packets. It is a further object of the invention to provide the mobile device with improved integrity determinations. It is a further object of the invention to closely align with open industry standards and operate across a variety of AGPS chipsets.